Though, they ended up writing a whole new Ruby JIT to deal with Ruby's performance, so clearly at some point the trade-off changes from "no systems programming, a highly dynamic language instead" to "hardcore systems programming, to make the highly dynamic language fast enough"
Shopify got to huge scale first and then started sponsoring TruffleRuby and YJIT as research projects.
I think the takeaway here is that it was more sensible to reimplement Ruby than to use something else. Think about that for a moment.
Instagram was also initially built in monolithic Django
I mean, if you can get performance for free, why not? Doesn't mean Ruby was too slow...